Overstuffing isn't high on my list of DCEU criticisms thus far. I didn't care for the JL cameos in BVS, but that was more due to the execution than the notion of the cameos. I think it could've worked more efficiently with better writing. And Doomsday kind of blew, but we always knew there had to be some sort of physical threat under Lex's thumb. As for Suicide Squad, my main issues were that the big bad sucked and the edit as a whole was questionable. I didn't really have any problem with the number of characters in play.
For a Flash solo, the Rogues make sense to me. Each season of the show has had a speedster as the big bad, so I can see why they'd like to avoid that. And I don't think an individual member of the Rogues really packs enough punch to be a solo villain in their own film. I like this. Maybe not every Rogue, but I could see Boomerang, Cold and Mirror Master. Or maybe Weather Wizard.